Detach primary CUDA context before deleting

[lohedges]

Somehow the CUDA cleanup method wasn't ported correctly when refactoring to add support for different GPU platforms. As such, the context was deleted before the context was detached, which could lead to a core dump file being written.

[lohedges]

Actually, the new approach is correct since we are using a shared primary context, so we don't want to bring all simulations down. Will have a think about what might be causing the cleanup issues.